How to Make It
In a medium saucepan, cover the beets with water. Add a pinch of salt and bring to a boil. Simmer the beets over moderate heat until tender, about 1 hour. Drain and let cool slightly. Peel the beets, then cut them into thick matchsticks and transfer to a bowl.
In a large bowl, whisk the vinegar with the mustard, shallot and tarragon. Whisk in the peanut and olive oils and season with salt and pepper.
Add 3 tablespoons of the vinaigrette to the beets and toss well. Add the mâche to the remaining vinaigrette and toss well. Add the beets and toss. Transfer the salad to plates and serve at once.